1. Field of the Invention
This invention relates to production of pinacolone (or tertiary-butyl methyl ketone).
2. Description of the Prior Art
It is well known in the field of organic chemistry that pinacolone can be produced via pinacol from acetone. The process involves the so-called pinacol-pinacolone rearrangement (cf. e.g. Organic Syntheses, Collective Volume 1, pages 459-463). The process comprises reacting acetone with metallic magnesium (or metallic aluminum) in the presence of mercuric chloride, hydrolyzing the resulting magnesium (or aluminum) salt of pinacol and treating the resulting pinacol with sulfuric acid to cause dehydration and said rearrangement. The case where metallic magnesium is used may be illustrated by the following equations (1)-(3): ##STR3##
In commercial scale production, however, this method is disadvantageous from economical and socio-environmental standpoints. The reasons, among others, are that: the fairly expensive metallic magnesium or aluminum used as one of the raw materials is converted into a useless salt; toxic mercuric chloride is required, and during the reaction the major part of it is converted into elemental mercury, which is very difficult to be isolated quantitatively from the reaction mixture; and acetone is employed in a large excess, and a part of this is reduced to give a large amount of isopropyl alcohol as a byproduct which necessitates the use of large quantities of energy to recover and purify the excess acetone. Because of potential environmental problems, it would be advantageous to provide a process with reduced use and formation of hazardous substances and reduced formation of industrial wastes.
An alternative method is also known which comprises hydrolyzing in the presence of a strong acid, 4,4,5-trimethyl-1,3-dioxane obtained by Prins reaction of 2-methylbutene-2 with formaldehyde (see German Patent No.714,488, Chemical Abstracts, vol. 78, 71330d (1973) and U.S. Pat. No. 4,059,634). The reactions involved may be illustrated by the following equations (4) and (5): ##STR4##
This process, also, has such drawbacks as: the decreased yield of pinacolone; the simultaneous formation of formaldehyde, followed by its consecutive consumption; the formation of a considerable amount of viscous byproduct; and the necessity of recovery or separation of the byproducts from pinacolone, whereby the reaction process becomes complicated and the purity of the product is decreased.
An improved method has been proposed to avoid such disadvantages, which method comprises adding formalin slowly to 2-methylbutene-2 or 2-methylbutene-1 or an acid adduct of either of these butenes in the presence of an inorganic acid to cause formation of pinacolone (cf. U.S. Pat. No. 4,057,583). This method provides pinacolone in increased yield in one reaction step, as compared with the method of the German Patent cited above.
However, the minimum inorganic acid concentration necessary for the reaction is at least as high as 15 weight %, and to obtain satisfactory results, it is necessary to use a large quantity of a highly concentrated acid. In the exemplary case of hydrochloric acid, which is said to give the best results, the minimum concentration applied in the examples of the U.S. Pat. No. 4,057,583 is 30 weight % and the amount required is 2 moles or more per mole of a 2-methylbutene. This means that a large amount of hydrochloric acid needs to be employed in high concentration for obtaining a satisfactory reaction yield and that the resulting pinacolone will be contaminated with a considerable amount of the acid, whereby various disadvantages are caused in purification of pinacolone. Moreover, the necessity of using highly concentrated hydrochloric acid causes various disadvantages in the commercial application of the process. Re-use of the aqueous acid solution is complicated because the acid, which is diluted during the reaction, is very difficult to reconcentrate by conventional distillation to a hydrogen chloride concentration beyond 20.24 weight %. Generally, aqueous hydrochloric acid solution forms an azeotropic mixture with water at this concentration. It is possible to increase the acid concentration by addition of hydrogen chloride, as described in the example of the U.S. Pat. No. 4,057,583; however, this procedure is very disadvantageous because it inevitably increases the total volume of acid.